Conventionally, an analog-to-digital converter (ADC) includes a comparator. The comparator is used to compare two input signals and then generate a digital signal (low or high) indicating which of the two input signals is bigger. Under normal comparison, the comparison time of the comparator should be within a predetermined time limit. If the two input signals are very close to each other, a comparison result can hardly be determined within the predetermined time limit, which leads to a very long comparison time.